1. Field of the Invention
The present invention relates generally to control circuits for fluorescent lamps. More particularly, the present invention relates to a low dimming antiflicker control circuit for a cold cathode fluorescent lamp.
2. Discussion of Related Art
Historically and currently, cold cathode fluorescent lamps (CCFLs) have been used to back light liquid crystal displays (LCDs). CCFLs are well suited to this application due to their low cost and high efficacy. High efficacy, which is equal to the ratio of light output to input power, is required because typical LCDs only transmit about 5% of the backlighting due to absorption of light in the polarizer and color filter of the LCD. In order to produce usable daytime lighting levels of approximately 400 Nits, the backlight must be capable of 20xc3x97400 Nits. One Nit is the luminance of one candle power measured one meter away over a meter by meter area, also known as a candela per meter squared. A cost effective backlighting technology which can provide such a lighting level is a fluorescent lamp.
Although the CCFL is an extremely efficient light source, it is difficult to control down to the low dimming levels required by, for example, night time automotive environments. In one automotive specification, the requirement for dimming is to a barely discernable level, which is in the range of 1.0 Nit for an active matrix LCD. Accordingly, the CCFL controller must be capable of producing a dimming ratio of 400:1.
Most CCFL controllers have difficulty in controlling the absolute luminance down to the level of imperceptibility. Some known systems obtain the desired dimming ratio by overdriving the lamp. However, this rapidly reduces the operating life of the lamp. Some military LCD systems use a first lamp for daytime illumination and a second, smaller lamp to produce the required night time lighting levels. However, systems which utilize dual lighting sources are not cost competitive in the automotive environment. Not only is a second lamp required, but a second controller is required as well.
Many control schemes have been used to control fluorescent lighting. Examples include voltage controlled self-resonant oscillators, pulse-by-pulse current pulse width modulated (PWM) control and PWM duty cycle control systems or combinations thereof. Pulse-by-pulse current PWM control systems characteristically operate at a frequency of 20 KHz to 100 KHz to control the lamp current. PWM duty cycle control of the CCFL luminance is accomplished by duty cycle control of the lamp""s on time to the total periodic update time. As an example of PWM duty cycle control, if the operational frequency of the CCFL driver is 60 KHz and, a periodic PWM update frequency of 2xc3x9760 Hz or 120 Hz is used, then an update time of 8.33 msec ({fraction (1/120)} Hz) results. In this example, there are a total of 500 (8.33 msecxc3x9760 KHz) lamp current drive cycles per update time. Therefore, if 50% luminance is desired, the CCFL only turns on the lamp for 250 out of the total possible 500 cycles for each update period. If the lamp were turned on for only 1 out of 500 cycles, the dimming ratio would be 500:1. However, practical lamps require several current pulses to start the flow of lamp current.
In order to obtain a cost effective dimming controller for automotive applications, a variation of a commercially available product must be used. Until recently, most controllers were variations of a self-resonant oscillator configuration which is sufficient for lap top personal computer (PC) applications, for example. Such controllers do not have the dimming control range required for automotive applications. However, a dimming solution being used more often is a direct drive (non-resonant) PWM controller. One example is the model LX1686 controller produced by LinFinity Microelectronics of Garden Grove, Calif. This controller features both PWM duty cycle and pulse-by-pulse lamp current PWM control. The cycle-by-cycle lamp current control is especially useful because the current level of each cycle can be either a low night-time value, a normal day-time value or a boosted value for rapid heating during cold weather conditions. Moreover, this controller is extremely cost competitive and therefore suitable for cost-sensitive automotive applications.
While this controller has substantial advantages, for some applications this controller has the disadvantage of being unable to control the minimum number of current pulses over temperature for the desired low luminance operation. A minimum number of current pulses is required each PWM duty cycle to prevent the plasma from extinguishing and requiring a restart operation which will cause the lamp to flicker. Accordingly, there is a need for an improved controller permitting accurate control of the minimum number of current cycle pulses, thereby allowing flicker free operation over the automotive temperature range.
By way of introduction only, a flicker reduction method for a lamp assembly includes providing current pulses to illuminate a fluorescent lamp in response to a periodic signal such as a ramp voltage. A feedback circuit samples current in the fluorescent lamp to ensure that a predetermined number of current pulses have been provided to the fluorescent lamp per period of the periodic signal. After the pulses are provided, the circuit is reset for the next cycle of the periodic signal.
The foregoing discussion of the preferred embodiments has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention.